Metal tubes are appreciated for their high restitution rate. Also the perfect plasticity of the metal prevents any of the product returning inside the tube when the operator has finished pressing the skirt of the tube. On the other hand, the metal tube is subjected to corrosion when it contains particularly acidic or alkaline products.
Attempts consisting in coating the inner surface of metal tubes with varnish have proved unsuccessful: the deposited coat is never very regular, it holds more or less well onto the surface, it is fragile and cracks after the tube has been used a number of times. Cracks appear through which the product passes and eventually corrodes the metal. Also, the ends of the tube, i.e. the neck at one end and the open end at the other through which the tube is filled and that is then flattened and folded to seal the tube, are not protected or are insufficiently protected by the varnish.
French patent application 2 322 058 discloses a double-walled tube constituted by two separate tubes that are connected at the necks. The metal tube constitutes an outer envelope that is protected from the corrosive product by a plastic inner tube. The top end of the neck of the inner plastic tube projects far beyond the top of the neck of the external metal tube. The projected section flares out in a trumpet shape above the metal neck and is then embedded into a plastic end molded above and around the metal neck. Three separate operations are required to produce a tube of this kind: the insertion of the inner tube into the outer tube, the flaring of the projected end of the neck of the inner tube and then the difficult molding of a plastic collar onto the edge of the outer metal neck.
Patent application J07277349 describes a double-walled tube that is also constituted by two separate tubes but that are connected together at the neck by a nozzle that is tightly threaded around the assembly. As in French patent application 2 322 058, the top end of the neck of the inner tube projects well beyond the top end of the outer metal tube. The projecting section is flared and then folded outwards covering the outer metal neck. The nozzle is tightly threaded and bears on the folded section of the inner tube. In order to ensure that the connection is leaktight J07277349 recommends that at least one surface of the ends of the inner tube, which is made of plastic, has a smooth surface without a mold joint line. A smooth surface of this kind is achieved by injection-molding the inner tube.
The end of the neck of the inner plastic tube must be thin enough in order for it to be folded over the top end of the neck and to cover the lateral outer surface of the metal neck. The thinness of the inner plastic tube means that it is prone to buckling and it does not facilitate the first shaping stage in which the neck is flared. This essential stage is complicated by the need to design tooling to hold the neck steady when it is flared. Also, with large capacity tubes that must have a skirt length of over 130 millimeters injection-molding cannot easily be used to produce a thin, plastic neck able to withstand cracking under strain. These tubes require materials to be used that are suitable for being injected in a long, narrow air gap. Materials of this kind must have a high melt index that is higher than 15 and they are much more prone to cracking under strain when they are brought into contact with the contained product than the materials generally used with a much lower melt index (0.2 to 2).
Also, it would be preferable to produce the inner plastic tube using other, more economical, means, for example using extrusion blow-molding or by molding a head onto an extruded or welded laminated skirt. In order to achieve this, the need to have an outer surface without any protrusions, such as a molding joint line, would have to be set aside, as would the need to obtain a neck shape that could be series-produced. Both these conditions are necessary in J07277349 in order to obtain complete leaktightness of the connection between the necks.